The present invention generally relates to methods and apparatuses for controlling optical output of a laser light source, and more particularly to a method and an apparatus for controlling an optical output of a laster light source in an image forming apparatus such as a laser printer having a laser power set mechanism.
With an increase in printing speed and printing density of the laser printer, a rotational speed of a laser beam deflector such as a polygonal mirror is becoming quite high. As a result, a scan time of one scanning line is becoming short, and a scan time of a non-photosensitive region within a laser scanning region is becoming short.
A reference clock signal of a laser power set mechanism which sets a laser power of a laser light source, that is, a counting speed of an up/down counter, is uniquely determined by a response of a power monitor part which monitors an optical output of the laser light source. A laser diode constituting the laser light source is turned ON during a laser power set operation, and in order to prevent a photosensitive body from becoming deteriorated, it is desirable that the power set operation is carried out within a scan time in which a non-photosensitive region is scanned. However, as described above, the scan time of the non-photosensitive region is becoming short due to the increase in both the printing speed and the printing density of the laser printer, and there is a problem in that the power set operation cannot be completed within the scan time of the non-photosensitive region. In other words, it is becoming more and more difficult to adjust or readjust the laser power to a predetermined value.
Conventionally, there is a method of carrying out a power set operation with respect to the laser light source by use of signals having timings shown in FIGS. 1(A) and 1(G). When a power set start signal c shown in FIG. 1(C) from a central processing unit (CPU, not shown) is supplied to an enable signal generator (not shown), an enable signal g shown in FIG. 1(G) is set to a high level (g1) and a laser diode signal f shown in FIG. 1(F) is set to a high level (f1) to turn the laser diode ON. When a monitor signal d shown in FIG. 1(D) changes from a low level to a high level (d1), an end signal e shown in FIG. 1(E) from an end signal generator (not shown) turns the laser diode OFF by setting the laser diode signal f to a low level (f2). At the same time, the enable signal g is also set to a low level (g2) to end the ON state of the laser diode.
In other words, the power set operation is carried out regardless of whether it is a high-level period of a gate signal b shown in FIG. 1(B) indicating a scan time of a photosensitive region or a low-level period of the gate signal b indicating a scan time of a non-photosensitive region. For this reason, there is a problem in that a specific portion of the photosensitive body becomes deteriorated by the exposure which takes place due to the power set operation carried out within one scanning line which is indicated by an interval between signals portions a1 and a2 of a synchronization detection signal a shown in FIG. 1(A).
Even when realizing the high-speed printing with the high printing density, it is important that the photosensitive body is exposed uniformly by the power set operation. But according to the conventional method, the exposure by the laser beam takes place within one scanning line at a specific portion of the photosensitive body and not uniformly on the photosensitive body because the power set operation is carried out regardless of whether the region being scanned is the photosensitive region or the non-photosenstive region.
In addition, even among laser printers of the same model but of different picture element densities, an optimum timing of the power set operation differs depending on the picture element density. That is, as the picture element density becomes high (the rotational speed of the deflector becomes high), the scan time of the non-photosensitive region becomes short and there is a problem in that the power set operation cannot be completed within this scan time of the non-photosenstive region.